Xenopus laevis is being studied as a vertebrate animal that offers certain advantages for the analysis of molecular events during development. The major approach currently underway is based on the preparation and analysis of an enriched cDNA library which represents those RNA molecules that are present in gastrulae but absent from the egg. Genes differentially expressed in gastrula (DG genes) are being used as a source of molecular markers of early development. A detailed analysis of accumulation of over 20 DG RNAs during development has been carried out, showing that these RNAs begin to accumulate at a distinct but closely spaced times in late blastula to early gastrula. Most RNAs decrease in concentration during or shortly after neurula. By dissection of neurula embryos the distribution of these DG RNAs has been tested. Most DG RNAs appear evenly distributed throughout the neurula, but four cases have been found which show substantial enrichment in the ventral or the posterior regions of the embryo. In situ hybridization is being used to study the distribution of DG RNAs in further detail. DG 42 represents a messenger RNA present only in gastrula/neurala stages, and the cDNA clone has been sequenced. A related cDNA clone, DG 21, has the same developmental profile but shares only 80% sequence homology with DG 42. Genomic clones encoding DG 42 and DG 21 have been isolated and mapped, and are currently being subjected to sequence analysis. Experiments are in progress to express portions of DG cDNAs in bacteria with the aim to produce polypeptides that will be used to produce antibodies against natural DG proteins. Peptides corresponding to parts of the predicted DG 42 protein sequence are being synthesized and will also be used for antibody production.